﻿11G 



SUPPLEMENT TO THE CRAG MOLLUSCA. 



Arca tetragona, Foli. Crag Moll., vol. ii, p. 76, Tab. X, fig. 1. 



Localities. Cor. Crag, Sutton, Ramsholt, and Sudbourn. Red Crag, Sutton. 



The specimen represented in figs. 1 a and b (' Crag Moll.') is from the Red Crag at 

 Sutton, and it was there considered as belonging to the above-named species ; figs. 1 c and 

 1 d are from the Coralline Crag. In the 'Brit. Conch.,' vol. ii, p. 181, A. tetragona is 

 stated to be in the Red and Cor. Crag, but in the list accompanying Mr. Prestwich's paper 

 this species {tetragona) is given from the Cor. Crag only, while my shell from the Red Crag 

 is referred by this author to A. imbricata, Poli. In consequence of this statement I have 

 again examined and compared my specimens with the Mediterranean shell, but must still 

 retain my previously formed opinion. 



Pectunculus glycimeris, Linn. Crag Moll., vol. ii, p. 66, Tab. IX, fig. 1. 



Localities. Cor. Crag passim. Red Crag passim. Fluvio-marine Crag, Bramerton. 

 Chillesford bed, Aldeby. Middle Glacial, Billockby and Hopton. Upper Glacial, 

 Bridlington. 



The genus Pectunculus is one in which the species are of very difficult determination, 

 as may be seen by the number of synonyms given by various authors, and the little 

 accordance there is as to specific separation. In the Cor. Crag the prevailing form is the 

 thick variety, corresponding in this respect with the Mediterranean shell or rather with the 

 Sicilian fossils, but I have also found in the Cor. Crag the thinner and less tumid shell, 

 like that of the recent British variety. In the ' Crag Moll.' I gave P. ghjcimeris and 

 P. pilosus as varieties of one species, following the authors of the ' Brit. Moll.,' and I see 

 no sufficient reason to alter that opinion. All I can say is that, if there be two species, 

 as appear to be made out of these two varieties by some authors, they are both present in 

 the Coralline Crag as well as in the Red. Mr. Canhain has obtained from the Red Crag 

 at Waldringfield a specimen which measured 3| of an inch in one direction and 

 3^ in the other, and this of great solidity, and with these are specimens thin and oblique, 

 while some have prominent umbones ; in others this part is much depressed, with every 

 intermediate shade of difference. It is difficult to say whether the thick shells in the Red 

 Crag are, like the thinner ones, natives of the Red Crag sea, or whether they be derived 

 from the Cor. Crag, as I think there can be no doubt that a large proportion of the 

 Peclunculi, which make up the mass of the thin bed at the base of the Red Crag, from 

 which the phosphatic nodules are extracted, have been thus derived. P. stellatus and P. 

 insubricus are, I believe, mere varieties of this species. P. glycimeris (I presume the thin 



